Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a power amplifier for driving an audio output signal to headphones or speakers.
In general, personal audio devices continue to be reduced in size, yet many users desire louder sound from these personal audio devices. This places physical size constraints on a battery for powering components of the personal audio devices at the same time audio subsystems of such personal audio devices are demanding more output power. With the desire for higher audio volumes and quality, often a boosted supply voltage higher than the battery voltage is generated in order to supply an audio amplifier and deliver more power to the speaker load. As more power is delivered to the speaker load, more strain is placed on the battery of a personal audio device.
A battery includes an output impedance, and thus heavy loading conditions on a battery can cause a battery's output voltage to drop. Such drop in output voltage may be more prominent when the battery has a low level of charge. The sudden voltage drop produced by this loading event has the potential to reduce the battery's output voltage to a point where certain subsystems on the device are no longer able to function properly. When the battery is in a weakened or lower charge state and the personal audio device offers no protection against such weakened or lower charge state, often the end result is the personal audio device resetting itself due to a low voltage condition. This self-reset condition may be displeasing to a user of the personal audio device and thus problematic for the provider of the personal audio device (e.g., manufacturer, vendor, reseller, or other provider in a chain of commerce). Such a condition or conditions similar thereto in which an unintentional voltage drop occurs are commonly referred to as “brownout” conditions.
Traditional approaches to mitigation of brownout conditions in personal audio devices have been reactive in nature, in that a reactive brownout reduction system typically identifies the occurrence of a battery voltage falling below a predetermined voltage threshold (e.g., configured by a user or a provider of the personal audio device) and reacts responsive to the battery voltage falling below such threshold. An example of such reaction is a reduction of audio volume in order to reduce an audio amplifier's load on the battery.
This reactive methodology is based on a concept that an undesirable event has already occurred to the battery supply, and thus the personal audio device quickly takes action to reduce loading in order to prevent brownout of the personal audio device. Subsystems other than the audio subsystem and powered by the battery supply may also react independently in order to reduce loading on the battery supply and allow it to return to a safe level in order to maintain functionality of more critical subsystems of the personal audio device. Such reactive approaches do little or nothing to prevent the audio subsystem, and in particular an audio amplifier, from being a cause of the battery supply falling to an undesirable level that may trigger a brownout condition. A reactive brownout reduction system typically has no knowledge of the audio content and by extension, no knowledge of actual power supply loading caused by an audio signal path. Instead, such existing systems typically assume that the loading of an output amplifier of the audio signal path is the source of the supply drop and blindly reduce loading of the output amplifier, even if it is not the main source of the reduction in power supply.
A reactive brownout reduction system requires a certain amount of time to react before the audio signal to the audio amplifier is attenuated. Once the voltage supply of the battery drops, it also takes an additional amount of time to attenuate the audio signal and allow the battery supply to return to a “safe” operating voltage. The cumulative initial reaction time, system response time, and the battery supply recovery time may cause the system to spend a significant amount of time below the preconfigured threshold voltage of the battery supply.
If the audio system, in particular the audio amplifier, is the primary cause of the battery supply drop, and the battery is in a weakened state, this reactive methodology also has the potential of getting into a state of operation where the audio volume is repeatedly attenuated and then allowed to gain back up. From a user's perspective, this can produce a “pumping” effect of the audio content, where audio volume repeatedly gets louder and softer, as the reactive brownout reduction system may put the reactive brownout response into a continual loop.